[webview_flutter] Add a backgroundColor option to the webview

[e-adrien]

This PR add an option to set the background color of the webview.

Original PR :
#3431

Fixes flutter/flutter#29300
Fixes flutter/flutter#37697

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[flutter-dashboard]

This pull request was opened against a branch other than master. Since Flutter pull requests should not normally be opened against branches other than master, I have changed the base to master. If this was intended, you may modify the base back to master. See the Release Process for information about how other branches get updated.

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[stuartmorgan]

LGTM!

@mvanbeusekom for second review

[mvanbeusekom]

LGTM with one very small nit (p.s. thank you for all the work on this).

[mvanbeusekom]

nit: documentation can be improved a little bit (similar to the Android specific version).

Suggested change

	/// The background color of the webview.
	///
	/// When null the platform's webview default background color is used.
	///
	/// By default `backgroundColor` is null.
	/// The background color of the [WebView].
	///
	/// When `null` the platform's webview default background color is used. By
	/// default [backgroundColor] is `null`.

[e-adrien]

Done.

It's nothing compare to the amount of time we save thanks to Flutter. 😅
Thank you very much.

[stuartmorgan]

Something strange happened here; somehow this was redirected from master to main, but I'm not seeing it on main either. I tried opening a new copy of the PR to merge to master, but it gets reset to main again.

It looks like there's some kind of infra issue here? @godofredoc

[stuartmorgan]

Filed flutter/flutter#94790 to try to figure out what's going on here so we can get this landed to the correct branch.